Heretofore, when the rolling bearing in which grease is enclosed is used in a high load-applied condition, a lubricating film of lubricating grease is liable to fracture. When the lubricating film fractures, metal contact occurs to generate a disadvantage that heat generation and frictional wear increase. Therefore by using grease containing an extreme-pressure agent (EP agent), the disadvantage is decreased.
As the use condition of the rolling bearing becomes severe, by improving the lubricating properties of grease and the resistance thereof to a load, it is necessary to prevent the occurrence of the metal contact caused by the fracture of the lubricating oil film. A roller bearing has a flange and at a flange portion thereof, a rolling element and a flange of a bearing ring make a sliding motion. Thus at the flange portion, the fracture of the lubricating oil film is liable to occur.
Solid lubricant-containing grease in which for 100 parts by weight of an adduct of melamine (iso) cyanurate, 5 to 1000 parts by weight of a solid lubricant selected from the group of polytetrafluoroethylene, molybdenum disulfide, and molybdenum dithiocarbamate (thereinafter abbreviated as MoDTC) is used is known (patent document 1). An organic bismuth compound-containing an extreme-pressure grease lubricant composition for use in the rolling bearing is also known (patent document 2). Grease containing the MoDTC and polysulfide to decrease wear is also known (patent document 3).
The bearing for use in the rolling stock is classified into the bearing for use in the axle and the bearing for use in the main motor. In the case of the bearing for use in the axle, both end portions of the axle is supported by a tapered roller bearing mounted on an underframe of the rolling stock. In the case of the bearing for use in the main motor, both output-side end portions of the rotary shaft of the motor are supported by a cylindrical roller bearing or a ball bearing mounted on the underframe of the rolling stock.
In a wheel-supporting apparatus supporting a non-driven wheel such as a front wheel of a rear wheel-driving type vehicle, two rolling bearings are mounted on an axle (knuckle spindle) provided on a steering knuckle, a flange is provided on an outside-diameter surface of an axle hub rotatably supported by the rolling bearings, and a braking drum of a braking apparatus and a wheel disk of the wheel are mounted, with stud bolts provided on the flange and with nuts engaging the stud bolts with screws.
A back plate is mounted on the flange provided on the steering knuckle so that the back plate supports a braking mechanism for imparting a braking force to the braking drum.
In the above-described wheel-supporting apparatus, as the rolling bearing rotatably supporting the axle hub, a tapered roller bearing having a high load capacity and a high rigidity is used. The tapered roller bearing is lubricated with grease enclosed between the axle and the axle hub.
As an example of the bearing for use in the rolling stock and the wheel-supporting apparatus to which a high load is applied at a high-speed operation, a bearing for use in the rolling stock in which grease containing not more than 20 wt % of an organic metal compound containing metal selected from among nickel, tellurium, selenium, copper, and iron for the entire amount of the grease is enclosed is known (patent document 4).
In a rolling neck bearing for use in a rolling machine, generally, an inner ring thereof has one double row inner ring, and an outer ring has one double row outer ring and two single row outer rings disposed at both ends of the double row outer ring via a spacer. Rolling elements are circumferentially rotatably disposed in four rows between the inner ring and the outer ring. An annular seal member is mounted on both end portions of the outer ring.
The rolling neck bearing for use in the rolling machine is used in a rolling process of a steel-manufacturing factory in an environment in which a rolling liquid containing water as its main component is jetted. Thus there is a problem that when water penetrates into the bearing, a lubricating oil film is fractured and the bearing is damaged at an early stage owing to inferior lubrication.
To cope with this problem, an example of the rolling neck bearing having the following construction is known: An annular seal member is mounted at both end portions of the outer ring, with a seal lip portion thereof in contact with the peripheral surface of the inner ring. An intermediate seal member is mounted at an inner peripheral side of a butted end of one double row inner ring. By forming a slit for a vent mechanism on the intermediate seal member, even though air inside the bearing expands or contracts owing a change of temperature, the difference between a pressure inside the bearing and a pressure outside the bearing is automatically balanced to prevent the water from penetrating into the bearing (patent document 5).
However, the rolling bearing having the above-described construction is used as the rolling neck bearing for use in the rolling machine, the seal lip portion of the annular seal member mounted on the outer ring at both axial ends thereof is of a type which makes a line contact with the peripheral surface of the inner ring. Thus in an environment in which the rolling roller is frequently mounted and removed for re-grinding, the seal lip portion is much damaged. Consequently rolling water and cooling water penetrate into the bearing at a rate of not more than 20% of the grease and mix with a lubricant (normally, grease lubricant in which lithium-based thickener such as Adlex, Albania or the like is used), thus deteriorating the lubricating function thereof and causing the bearing to be damaged in an early stage owing to inferior lubrication.
Regarding the rolling bearing unit for supporting the wheel of a car, a rolling bearing of a first example having the construction in which the inner ring is set as the stationary-side bearing ring and the hub is set as the rotational-side bearing ring is known. A rolling bearing of a second example having the construction in which the outer ring is set as the stationary-side bearing ring and the hub is set as the rotational-side bearing ring is known (patent document 6).
The first example of the conventional construction of the wheel-supporting rolling bearing unit is described below with reference to FIG. 12. FIG. 12 is a sectional view showing the first example of the conventional construction of the wheel-supporting rolling bearing unit. The wheel 1 is rotatably supported by the wheel-supporting rolling bearing unit 2 as shown in FIG. 12 at the end portion of the shaft 3 constructing the suspending construction. That is, the inner rings 5, 5, set as the stationary-side bearing ring, which construct the wheel-supporting rolling bearing unit 2 are fitted on the axle 4 fixed to the end portion of the shaft 3 and fixed thereto with the nut 6. The wheel 1 is fixedly connected to the hub 7, set as the rotational-side bearing ring, which constructs the wheel-supporting rolling bearing unit 2 with a plurality of the studs 8, 8 and the nuts 9, 9.
The double row outer ring rolling surfaces 10a, 10b each serving as the rotational-side rolling surface are formed on the inner peripheral surface of the hub 7, and the mounting flange 11 is formed on the peripheral surface thereof. The wheel 1 and the drum 12 for constructing the braking apparatus are fixedly connected to one side surface (outer side surface in the illustrated example) of the mounting flange 11 with the studs 8, 8 and the nuts 9, 9.
In the specification, “outside” in the axial direction means the outer side in the widthwise direction in a state in which the bearing unit is mounted on a vehicle, whereas “inside” means the central side in the widthwise direction.
Between the outer ring rolling surfaces 10a, 10b and between the inner ring rolling surfaces 13a, 13b formed on the peripheral surfaces of the inner rings 5, 5 as the stationary-side rolling surfaces, a plurality of the balls 14, 14 which are the rolling elements respectively are rotatably provided, with the balls 14, 14 held by the cages 15, 15 respectively. By combining the constituent members with one another in this manner, a double row ball bearing of an angular type is constructed in a back-to-back arrangement, and the hub 7 is rotatably supported on the periphery of each of the inner rings 5, 5, and a radial load and a thrust load are freely supported. The seal rings 16a, 16b are provided between the inner peripheral surfaces of both end portions of the hub 7 and the peripheral surface of the end portion of each of the inner rings 5, 5 to disconnect the space in which the balls 14, 14 are provided and the inner space 17 from each other.
The open portion at the outer end of the hub 7 is closed with the cap 18.
When the above-described wheel-supporting rolling bearing unit 2 is used, as shown in FIG. 12, the axle 4 on which the inner rings 5, 5 are fixedly fitted is fixed to the shaft 3, and the wheel 1 and the drum 12 with which an unshown tire is combined are fixed to the mounting flange 11 of the hub 7. The drum 12, and an unshown wheel cylinder and an unshown shoe both supported by the backing plate 19 fixed to the end portion of the shaft 3 are combined with one another to construct the drum brake for braking use. At a braking time, a pair of shoes provided at an inside-diameter side of the drum 12 is pressed against the inner peripheral surface of the drum 12. Grease is enclosed inside the inner space 17 to lubricate the rolling contact portion among the outer ring rolling surfaces 10a, 10b, the inner ring rolling surfaces 13a, 13b, and the rolling surfaces of the balls 14, 14.
The second example of the conventional construction of the wheel-supporting rolling bearing unit is described below with reference to FIG. 13. FIG. 13 is a sectional view showing the second example of the conventional construction of the wheel-supporting rolling bearing unit. In the case of the wheel-supporting rolling bearing unit 2a shown in FIG. 13, the hub 7a serving as the rotational-side bearing ring is rotatably supported by a plurality of the balls 14, 14 each serving as the rolling element at the inside-diameter side of the outer ring 20 serving as the stationary-side bearing ring. To do so, the double row outer ring rolling surfaces 10a, 10b each serving as the stationary-side rolling surface are formed on the inner peripheral surface of the outer ring 20, and the first and second inner ring rolling surfaces 21, 22 each serving as the rotational-side rolling surface are formed on the peripheral surface of the hub 7a. The hub 7a is constructed in combination of the hub body 23 and the inner ring 24. The mounting flange 11a for supporting the wheel is provided at the outer end of the peripheral surface of the hub body 23. The first inner ring rolling surface 21 is formed at the intermediate portion of the hub body 23. The small-diameter stepped portion 25 whose diameter is smaller than that of the portion where the first inner ring rolling surface 21 is formed is provided at the portion near the inner end of the intermediate portion of the hub body 23. The inner ring 24 having the sectionally circular arc-shaped second inner ring rolling surface 22 formed on the peripheral surface thereof is fitted on the small-diameter stepped portion 25. Further the inner end surface of the inner ring 24 is held down by the caulking portion 26 formed by plastically deforming the inner end portion of the hub body 23 radially outward to fix the inner ring 24 to the hub body 23.
Seal rings 16c, 16d are provided between the inner peripheral surfaces of both end portions of the outer ring 20 and the peripheral surface of the intermediate portion of the hub body 23 as well as the peripheral surface of the inner end portion of the inner ring 24 to disconnect the inner space 17a in which the balls 14, 14 are provided and the outer space from each other between the inner peripheral surface of the outer ring 20 and the peripheral surface of the hub 7a. 
Grease is enclosed in the inner space 17a to lubricate the rolling contact portion among the outer ring rolling surfaces 10a, 10b, the inner ring rolling surfaces 21, 22, and the rolling surfaces of the balls 14, 14.
In the lubrication of the rolling bearing portion, to prevent the lubricating film of the lubricating grease from being fractured, the grease containing the extreme-pressure agent (EP agent) is used to reduce the fracture of the lubricating oil film.
For example, the extreme-pressure grease lubricant composition containing the organic bismuth compound for use in the rolling bearing is known (patent document 2). The grease containing the MoDTC and polysulfide to reduce wear is also known (patent document 3).
It cannot be said that the constant velocity joint is satisfactory in a severe operation condition generated in recent high-performance cars. Any of the constant velocity joint of a double off-set type, the constant velocity joint of a cross groove type, and the like used as the constant velocity joint of the plunging type and a bar field joint used as the constant velocity ball joint of a stationary type has a construction of transmitting a torque by means of several balls. In these constant velocity joints, owing to a reciprocating motion of complicated rolling and sliding under a high surface pressure during rotations thereof, a stress is repeatedly applied to balls and metal surfaces that contact the balls. Thus a flaking phenomenon is liable to occur owing to metal fatigue. Because a car is lightweight in recent years so that an engine is capable of producing a high output and fuel expenses can be decreased, the constant velocity joint is small-sized. Thus the constant velocity joint is subjected to a relatively high surface pressure. Consequently the conventional grease is incapable of sufficiently preventing the occurrence of the flaking phenomenon. Further it is necessary to improve the heat resistance of the grease.
Heretofore to prevent the fracture of the lubricating film of the lubricating grease, the extreme-pressure agent (EP agent)-containing grease is used for the above-described grease for use in the constant velocity joint to decrease the fracture of the lubricating oil film of the lubricating grease.
For example, the grease (patent document 7) obtained by mixing an organic molybdenum compound with urea-based grease and the grease (patent document 8) obtained by mixing molybdenum disulfide, the MoDTC, and a sulfur-containing organic tin compound with the urea-based grease are known.
Because the rolling bearing and the constant velocity joint are used in a severe condition of a high speed and under a high load, the flange of the bearing ring makes a sliding motion on the large end face of the roller and the flange portion thereof. Thus the lubricating oil film of the lubricating grease is liable to fracture. Owing to the fracture of the lubricating oil film, metal contact occurs to generate a disadvantage that heat generation and frictional wear increase.
Therefore by improving the lubricating properties of the grease and the withstand load at a high speed and under a high load, it is necessary to prevent the occurrence of the metal contact caused by the fracture of the lubricating oil film. Thus by using the extreme-pressure agent-containing grease, its disadvantage is decreased.
The rolling bearing and the constant velocity joint are subjected to rolling friction between the rolling surface of the inner ring as well as the rolling surface of the outer ring and a “roller” which is a rolling element and subjected to sliding friction between the flange portion and the “roller”. Because the sliding friction is larger than the rolling friction, seizing of the flange portion is liable to occur when the use condition becomes severe. Therefore the conventional rolling bearing and the conventional constant velocity joint have a problem respectively that they cause a grease-exchanging work to be frequently performed and are incapable of making maintenance free.
Further as the use condition of the rolling bearing and that the constant velocity joint become severe, for example, when they are lubricated at a high speed of not less than 100,000 in dN value, the conventional grease is incapable of preventing the occurrence of the flaking phenomenon sufficiently. Thus it is difficult to use the constant velocity joint.
In the case of the conventional rolling parts and the rolling bearing, when the grease-enclosed rolling bearing is used under a high load, the lubricating oil film of the lubricating grease is liable to fracture. When the lubricating oil film has fractured, metal contact occurs to generate a disadvantage that heat generation and frictional wear increase. Thus the grease containing the extreme-pressure agent (EP agent) is used to reduce the disadvantage of the lubricating oil film.
To provide a sliding member or a rolling member and a rolling bearing having a low friction, a low wear, and sufficiently improved withstand load and seizing, base grease is allowed to chemically react with at least one of an organic phosphor compound, an organic sulfur compound, an organic chlorine compound, and an organic metal compound. Thereby the sliding member or the rolling member having a film layer formed by the reaction of compounds in a thickness of 0.05 to 0.5 μm as a result of the chemical reaction is obtained (patent document 9).
To improve the frictional characteristics of rolling parts in a boundary lubrication condition so that the rolling parts have little variations in the frictional characteristics thereof and have a stable and long life in the bearings thereof, a rolling part on which a coating film of metal salts of thiophosfate is formed is known (patent document 10).
But these rolling parts have a problem that they are insufficient in wear-decreasing effect on sliding surfaces thereof and lack long-term durability in the use condition of a high temperature and a high speed.
In the grease-enclosed rolling bearing, as the use condition of the rolling bearing becomes severe, it is necessary to prevent the occurrence of the metal contact caused by the fracture of the lubricating oil film by improving the lubricating properties of grease and the resistance thereof to a load. The roller bearing has in particular the flange, and at the flange portion, the rolling element and the flange of the bearing ring make a sliding motion respectively. Thus the grease-enclosed rolling bearing has a problem that at the flange portion, the fracture of the lubricating oil film is liable to occur.
Patent document 1: Japanese Patent Application Laid-Open No.61-12791
Patent document 2: Japanese Patent Application Laid-Open No.8-41478
Patent document 3: Japanese Patent Application Laid-Open No.10-324885
Patent document 4: Japanese Patent Application Laid-Open No.10-17884
Patent document 5: Japanese Patent Application Laid-Open No.2000-104747
Patent document 6: Japanese Patent Application Laid-Open No.2001-221243
Patent document 7: Japanese Patent Application Laid-Open No.63-46299
Patent document 8: Japanese Patent Application Laid-Open No.10-183161
Patent document 9: Japanese Patent Application Laid-Open No.2-256920
Patent document 10: Japanese Patent Application Laid-Open No.11-30236